1. Field of the Invention
The present invention relates to a rotation angle detecting apparatus that includes an incremental encoder for detecting rotation angle of a synchronous rotary electric machine such as a synchronous ac generator or a synchronous motor.
2. Description of the Related Art
Such an incremental encoder is a relative rotation angle detector that detects an index pulse generated each rotation of a rotor of a rotary machine, thereby detecting a rotation angle of the rotor relative to the stator of the rotary machine.
However, it is impossible to know an incremental rotation angle or an absolute rotation angle of the rotor by the incremental encoder, after electric power supply has been cut, until the power supply resumes and an index pulse is generated.
Therefore, a plurality of phase coils (e.g. U-coil, V-coil, W-coil of a three-phase winding) of a rotary electric machine have to be alternately powered at certain nonsynchronous time intervals until the index pulse is generated after power supply resumes. Because the index pulse is generated only once each rotation, only way to start the synchronous operation is to wait until the rotor makes one turn (360 degrees), at the maximum, after power is supplied to the rotary electric machine.